Change various helper classes to transition types to get CSSValue entirely onto the gc heap.

Source/core/css/CSSImportRule.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2005, 2006, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 22 matching lines @@
 CSSImportRule::CSSImportRule(StyleRuleImport* importRule, CSSStyleSheet* parent)
     : CSSRule(parent)
     , m_importRule(importRule)
 {
 }
 
 CSSImportRule::~CSSImportRule()
 {
     if (m_styleSheetCSSOMWrapper)
         m_styleSheetCSSOMWrapper->clearOwnerRule();
+#if !ENABLE(OILPAN)
+    // Oilpan automatically handles cycles and doesn't want destructors to touch potentially

[Mads Ager (chromium), 2014/03/03 10:38:45]

Maybe the comment should state that the MediaList and its parent CSSImport rules
died together with oilpan and therefore the clearing is not necessary?

[wibling-chromium, 2014/03/03 10:45:08]

Done.

+    // dead objects to perform cleanup.
     if (m_mediaCSSOMWrapper)
         m_mediaCSSOMWrapper->clearParentRule();
+#endif // ENABLE(OILPAN)
 }
 
 String CSSImportRule::href() const
 {
     return m_importRule->href();
 }
 
 MediaList* CSSImportRule::media() const
 {
     if (!m_mediaCSSOMWrapper)
@@ skipping 29 matching lines @@
         m_styleSheetCSSOMWrapper = CSSStyleSheet::create(m_importRule->styleSheet(), const_cast<CSSImportRule*>(this));
     return m_styleSheetCSSOMWrapper.get();
 }
 
 void CSSImportRule::reattach(StyleRuleBase*)
 {
     // FIXME: Implement when enabling caching for stylesheets with import rules.
     ASSERT_NOT_REACHED();
 }
 
+void CSSImportRule::trace(Visitor* visitor)
+{
+    visitor->trace(m_importRule);
+    visitor->trace(m_mediaCSSOMWrapper);
+    CSSRule::trace(visitor);
+}
+
 } // namespace WebCore